His past is completely unknown to him. His parents died in a car crash when he was very young, and because of his injuries, he has blocked out the memories of his past life. This makes him vulnerable to mental attacks by the Dark Kingdom, and other enemies. In the beginning, he was unaware he would transform into Tuxedo Mask. He would get terrible headaches and blackout whenever Sailor Moon was in danger. During those blackouts is when he would turn into Tuxedo Mask. He had dreams about a princess urging him to help her and find the Silver Imperium Crystal. In order to obtain it he needed to find the seven Rainbow Crystals. Sailor Moon and the Dark Kingdom were also searching for this crystals. He was severely injured protecting Usagi from Zoisite. It was during this time they both revealed their alter egos to each other, and when Mamoru realizes Usagi is the reincarnation of Princess Serenity, and he is the reincarnation of Prince Endymion.

To read more on Mamoru in other Sailor Moon seasons, check out Wikipedia.